Non-aqueous electrolyte batteries are roughly classified into liquid electrolyte type batteries and solid type batteries. Liquid electrolyte type batteries are batteries including a liquid electrolyte between the positive electrode and the negative electrode. While liquid electrolyte type batteries have high battery capacity, they need a precise structure to prevent a “leak”, which is an outward leakage of the liquid electrolyte from the battery. Solid type batteries are batteries including a solid electrolyte between the positive electrode and the negative electrode. Since solid type batteries are free from the fear of a leak, they have high battery safety and reliability. They also have advantages in that battery thickness can be reduced and battery lamination is possible.
In solid type batteries, various inorganic and organic materials are used as the solid electrolyte. Among them, solid electrolytes comprising inorganic materials have high ionic conductivity, but have high brittleness, which makes it difficult to form a flexible film. As solid electrolytes comprising organic materials, for example, polymer electrolytes comprising organic polymer compounds are known.
Polymer electrolytes, which are highly flexible and permit relatively easy formation of a thin film, compared to solid electrolytes comprising inorganic materials, are examined toward practical utilization. Among them, dry polymer electrolytes, which contain no non-aqueous solvent and thus have very high safety in addition to the aforementioned characteristics of polymer electrolytes, are expected to be put into practical utilization. Solid type batteries including a dry polymer electrolyte are commonly termed all solid-state polymer batteries.
A known example of dry polymer electrolytes is a composite of polyethylene oxide and an alkali metal salt such as a lithium salt or sodium salt. This dry polymer electrolyte, however, has a low ionic conductivity at room temperature of 10−4 to 10−7 S/cm. Hence, all solid-state polymer batteries including this dry polymer electrolyte are low in battery capacity, in particular, battery capacity under a high load.
In view of the above-stated problem with dry polymer electrolytes, examinations have been made as to various constituent components of all solid-state polymer batteries, such as the dry polymer electrolyte, active material, and electrode assembly structure. A proposal to improve the ionic conductivity of a dry polymer electrolyte is, for example, to make polyethylene oxide amorphous (for example, see Non-Patent Document 1). Specifically, making it amorphous means making the regular molecular arrangement in the crystal random, by linking the side chains of polyethylene oxide with short ethylene oxide chains.
However, the ionic conductivity of amorphous polyethylene oxide is only approximately 10−4 S/cm at room temperature, which is an insufficient improvement in ionic conductivity. Thus, the use of amorphous polyethylene oxide cannot solve the problem of the low battery capacity of all solid-state polymer batteries. In particular, after storage, internal resistance increases and battery capacity lowers significantly.
Also, there has been proposed a polymer electrolyte which is prepared by impregnating a polymer including polyvinylidene fluoride as the matrix with a solution that is prepared by dissolving a lithium salt in an ether such as diethoxyethane or dimethoxyethane (for example, see Patent Document 1). Patent Document 1 intends to improve the electrical conductivity, i.e., ionic conductivity of a polymer electrolyte by impregnating a polymer with an ether solution of a lithium salt.
There has also been proposed a polymer electrolyte which is prepared by hydrosilylation of a copolymer of vinyl ether and allyl vinyl ether and subsequent cross-linking in a liquid plasticizer in the presence of a diprotic cross-linking agent (for example, see Patent Document 2). The liquid plasticizer is dimethoxyethane, an oligo ethylene glycol dialkyl ether, or a polyethylene glycol dialkyl ether. The technique of Patent Document 2 intends to improve the ionic conductivity of a polymer electrolyte by providing a polymer electrolyte comprising a cross-linked polymer impregnated with a liquid plasticizer.
The polymer electrolytes of Patent Documents 1 and 2 have higher ionic conductivities than a dry polymer electrolyte comprising a composite of polyethylene oxide and an alkali metal salt. However, these improvements in ionic conductivity are not on the satisfactory level.
Dry polymer electrolytes have low ionic conductivities, as described above. Also, dry polymer electrolytes have poor flowability. Therefore, in all solid-state polymer batteries, the contact area of an active material layer and the electrolyte at the electrode interface becomes small. In particular, when lithium or a lithium alloy (hereinafter referred to as a “lithium-based active material”) is used as a negative electrode active material, the volume of the negative electrode active material layer changes significantly during charge/discharge. Further, the dry polymer electrolyte is subject to decomposition, and an insulating coating film is likely to be formed at the interface between the negative electrode active material layer and the electrolyte. It is therefore very difficult to prevent the contact area of the negative electrode active material layer and the electrolyte from becoming small. As a result, the internal resistance of the battery increases. As used herein, “electrode interface” refers to the interface between an active material layer and an electrolyte.
Due to such characteristics of dry polymer electrolytes, the diffusion of lithium ions at the electrode interface determines the rate of the charge/discharge reaction of all solid-state polymer batteries. Hence, in all solid-state polymer primary batteries, shortage of lithium ions capable of contributing to the electrode reaction at the electrode interface makes the electrode reaction difficult. As a result, polarization during charge/discharge increases, and discharge capacity under a high load decreases sharply. Also, in all solid-state polymer secondary batteries, repeated charge/discharge further decreases the contact area of the active material layer and the electrolyte, thereby lowering battery capacity, which eventually results in deterioration in charge/discharge cycle characteristics.
In order to obtain a highly reliable battery that is free from an internal short-circuit, it has been proposed to provide a thin layer (thickness 5 to 10 μm) of crystalline lithium nitride (Li3N) on the face of a lithium negative electrode facing a dry polymer electrolyte (for example, see Patent Document 3). Also, crystalline lithium nitride with relatively high ionic conductivity has been proposed (see Non-Patent Document 2). The ionic conductivity of the crystalline lithium nitride of Non-Patent Document 2 is 1.2×10−3 S/cm for single crystal in the direction perpendicular to c axis and 1×10−5 S/cm for monocrystal in the direction parallel to c axis. Also, it is 7×10−4 S/cm for polycrystal. However, even when the crystalline lithium nitride of Non-Patent Document 2 is used in the technique of Patent Document 3, it is difficult to suppress an increase in the internal resistance of the all solid-state polymer battery after storage.
Also, in order to enhance the coulombic efficiency of liquid electrolyte type non-aqueous electrolyte secondary batteries, it has been proposed to use a negative electrode including an alkali metal crystal with an average crystal grain size of 20 μm or more (for example, see Patent Document 4). When this negative electrode is used, the alkali metal deposits in the shape of spheres or thick lines on the surface of the alkali metal crystal during charge. Since most of the deposited alkali metal dissolves during discharge, coulombic efficiency improves. Likewise, the use of a negative electrode including an alkali metal crystal with an average crystal grain size of 1 μm or more can also improve the coulombic efficiency of the non-aqueous electrolyte secondary battery (for example, see Patent Document 5).
Although the negative electrodes of Patent Documents 4 and 5 are effective for liquid electrolyte type batteries, they increase the internal resistance before and after charge/discharge when used in all solid-state polymer batteries. It is thus not possible to sufficiently prevent battery capacity from lowering. Also, when an all solid-state polymer battery including the negative electrode of Patent Document 4 or 5 is produced as a secondary battery, the cycle characteristics become insufficient.    Non-Patent Document 1: Advanced Technologies for Polymer Battery II, edited by Kiyoshi Kanamura, p 113, CMC Publishing Co., Ltd    Non-Patent Document 2: Solid Ionics, coauthored by Tetsuichi Kudo and Kazuo Fueki, p 76, Kodansha    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 9-219218    Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 7-029413    Patent Document 3: Japanese Laid-Open Patent Publication No. Sho 63-298980    Patent Document 4: Japanese Laid-Open Patent Publication No. Sho 63-143747    Patent Document 5: Japanese Laid-Open Patent Publication No. Sho 63-146355